We hypothesize that signal peptide peptidase (SPP) is a superb drug target for pathogenic protozoans, and in particular Plasmodium species that cause malaria. We have obtained data showing that inhibitors of human SPP inhibit P. falciparum SPP (mSPP) and that these SPP inhibitors are capable of killing not only P. falciparum but also other protozoal parasites, T. gondii and T. brucei. Herein we propose to extend these observations to provide the rationale that will spur development of selective P. falciparum SPP (mSPP) inhibitors and provide the tools and infrastructure needed to develop those selective inhibitors. SPP is one member of a family of intramembrane cleaving, aspartyl proteases. In humans SPP and its homologs cleave the transmembrane domain of type II membrane proteins. In contrast, the presenilins (PSs), which are related intramembrane cleaving, aspartyl proteases cleave the transmembrane domain of type I membrane proteins. SPP appears capable of functioning as a protease with out any protein co-factors, whereas PS is catalytically active as a multi-protein complex known as [unreadable]-secretase. PSs have been intensively studies as therapeutic targets in Alzheimer's disease (AD). Indeed, nearly every major pharmaceutical company has had a substantial [unreadable]-secretase inhibitor (GSI) discovery program, and several GSIs are now in human trials. Notably some GSIs are capable of inhibiting the activity of human SPP and mSPP. Humans and other vertebrates express multiple homologues of SPP, while many protozoan parasites such as P. falciparum. T. gondii, C. parvum, G. lamblia, and E. histolytica contain a single SPP gene. Knockout or knockdown of a single SPP gene in C.elegans, Drosophila or Zebrafish results in an embryonic lethal phenotype. Thus, the observation that the protozoan pathogens contain a single member of the SPP protein family offers a compelling reason to hypothesize that inhibition of SPP in these organisms would be lethal and therefore an attractive therapeutic target for a broad range of pathogenic protozoans. Indeed, this hypothesis is strongly supported by our preliminary data. To validate SPP as a viable protein for future antiprotozoal development with a specific focus on the malarial parasite P. falciparum, we will utilize a multifaceted approach using biological, genetic and chemical techniques.